Journal of Volcanology and Geothermal Research 251 (2013) 5–15

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Journal of Volcanology and Geothermal Research

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Geometry and kinematics of the fault systems controlling the unstable flank of Etnavolcano (Sicily)

R. Azzaro ⁎, A. Bonforte, S. Branca, F. GuglielminoIstituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Piazza Roma 2, 95123, Catania, Italy

⁎ Corresponding author.E-mail address: azzaro@ct.ingv.it (R. Azzaro).

0377-0273/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.jvolgeores.2012.10.001

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 January 2012Accepted 1 October 2012Available online 9 October 2012

Keywords:FaultsSeismotectonicsGround deformationGeodynamic modelFlank instabilityMt. Etna

An updated tectonic framework of Etna's unstable flank has been defined as a result of multidisciplinary analysescarried out by integrating geological and geophysical data. The different typologies of datasets have been ana-lyzed and correlated in order to constrain the geometry and kinematics of the fault systems controlling the unsta-bleflank of Etna volcano and to better understand their complex relationshipwith the offshoremorphostructuresof the continental margin. In particular, we have considered as the main structural elements the following fourfault systems: Pernicana, Ragalna, Tremestieri–Trecastagni and Timpe. Slip-rates and kinematics have been esti-mated in both long- and short-terms, respectively, from geological and seismotectonic/geodetic data. Data inte-gration has allowed definingfive kinematic domains in the sliding flank of Etna: (1) theNEblock, bordered by thePernicana fault and characterised by the highest deformation velocities; ground velocity progressively diminishestoward South, with a clockwise rotation of the vectors defining (2) the block embracing the central part of theTimpe system; (3) the Giarre wedge; (4) the Medium-East block, bounded by the S. Tecla and Trecastagni faults;and (5) the SE block bordered, by the hidden Belpasso-Ognina tectonic lineament. The dynamics of these blockstakes place through discontinuousmovements: sudden short-term accelerations related to themagma intrusionare superimposed to a fairly constant mid-term ESE sliding. The proposed comprehensive model of the unstableflank provides the basic input parameters for applying analytical models to flank dynamics of Etna volcano.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

The geological evolution and volcano-tectonic features of volcanicedifices are strongly conditioned by the structural setting of theirbasement and by regional tectonics (see the special volume editedby Tibaldi and Lagmay, 2006, and papers therein). In particular, therelationships between volcano dynamics and regional stress regimeplay an important role in the magma transport from the mantle tothe surface and flank instability of volcanic edifices, with importantimplications for their geologic hazard (Nakamura, 1977; McGuire etal., 1997; Pasquaré and Tibaldi, 2003; Norini and Lagmay, 2005;Tibaldi, 2005; Tibaldi et al., 2005; Acocella, 2006).

Mount Etna is a large polygenetic basaltic volcano built up over thepast 500 ka on the eastern coast of Sicily in a geodynamic setting gener-ated during the Neogene convergence between the African and Europe-an plates (Lentini et al., 2006; Branca et al., 2011). The structuralframework of the Etna edifice is the result of a complex interaction be-tween regional tectonics, flank instability processes and basement ge-ometry (McGuire and Pullen, 1989; Borgia et al., 1992; Lo Giudice andRasà, 1992; McGuire et al., 1996; Montalto et al., 1996; Rasà et al.,1996; Rust and Neri, 1996; Monaco et al., 1997; Bousquet andLanzafame, 2004; Neri et al., 2005; Rust et al., 2005; Norini and

rights reserved.

Acocella, 2011). In particular, recent data obtained from different geo-physical techniques (Lundgren et al., 2004; Puglisi and Bonforte, 2004;Rust et al., 2005; Walter et al., 2005; Solaro et al., 2010; Bonforte et al.,2011) have highlighted that the unstable flank of Etna is dismemberedinto different blocks, characterised by homogeneous kinematics.

An updated structural setting of Etna volcano has recently been de-fined in the new volcano-tectonicmap, at 1:100.000 scale (Azzaro et al.,2012a) and reported by Barreca et al. (2012) in aGIS database. Themainstructural lineaments of the unstable flanks of Etna, are the followingfault systems (Fig. 1): Pernicana, Ragalna, Tremestieri–Trecastagni,Timpe and Ripe della Naca–Piedimonte–Calatabiano.

The Pernicana fault system (PF) is a transtensive structure with leftstrike-slip extending from Piano Provenzana in the upper northeastflank down to the Ionian coast. This complex transtensive structure isformed by a main E–W striking segment showing a south-facing scarp,which in the eastern portion is characterised by a purely left-lateral dis-placement rate of 2.8 cm/a in the last decade (Groppelli and Tibaldi,1999; Azzaro et al., 2001) and about 2 cm/a reconstructed over thepast 150 years by Rasà et al. (1996). The eastward propagation of thePF consists of a set of en-échelon right-stepping, ESE-striking segmentsextending down to the coastlinewhere the fault segments strike towardNNE. This fault system also includes the Fiumefreddo fault, an E–Wtrending normal fault with left-lateral strike-slip localized close to theIonian coast (Azzaro et al., 1998; Tibaldi and Groppelli, 2002). The off-shore continuation of the PF is a set of small NNE–SSW scarps, with

Fig. 1. Tectonic sketchmapof Etna volcano definedby structureswithmorphological expression (modified byBranca et al., 2011). Fault abbreviations: PF, Pernicana; RF, Ragalna; CF, Calcerana;TMF, Tremestieri; TCF, Trecastagni; FF, Fiandaca; ARF, Acireale; STF, S. Tecla; MF, Moscarello; SLF, S. Leonardello; RF, Ragalna; RP, Ripe della Naca; PD, Piedimonte; CL, Calatabiano. The arrowsindicate the strike-slip component, the contour of the rift zones is in red; inset map shows the location of the study area.

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few meters of vertical displacement, whose morphological evidence isconfined within 2 km from the coastline (Chiocci et al., 2011; Argnaniet al., 2012).

The Ragalna fault system (RF) is a dextral transtensive structure, lo-cated in the lower western flank of Etna (Fig. 1). This fault system,interpreted as the western boundary of the unstable south flank (Neriet al., 2007), is formed by a main segment, N–S striking, showing aneast-facing scarp with slip-rates of 3.4 and 3.7 mm/a calculated forleft-lateral and dip-slip components of movement (Neri et al., 2007).

The Tremestieri (TMF)–Trecastagni (TCF) is a normal fault system(Fig. 1) formed by two faults with right-lateral component representingthe southern boundary of the unstable east flank (Lo Giudice and Rasà,1992; Solaro et al., 2010). TMF is the longer NNW–SSE oriented seg-ment. Conversely, the TCF is a short segment, NW–SE oriented.According to Chiocci et al. (2011) the TMF–TCF system is linked witha main transcurrent fault located in the offshore of Acitrezza villagedissecting the laccolith of Ciclopi Islands.

The Timpe is a normal fault system with a right-lateral componentdissecting the lower eastern flank (Fig. 1). This fault system is formedby several main master segments named Fiandaca, Acireale, S. Tecla,Moscarello and S. Leonardello faults, showing slip-rates ranging from1.0 to 2.7 mm/a (Azzaro, 2004). In particular, the Fiandaca fault (FF) is

roughly N–S oriented and rotates toward SE near Acitrezza. The Acirealefault (ARF), roughly N–S trending, forms a scarp up to 120 mhigh alongthe coast while the S. Tecla fault (STF) is NW–SE oriented, forming ascarp up to 180 m high and in the south tip it is linked with ARF. TheMoscarello fault (MF), NNW–SSE trending, is the most prominentscarp of the Timpe system; finally, the S. Leonardello fault (SLF) formsa NNW trending graben as a consequence of the eastward extensionof this fault system. The offshore continuation of the Timpe system ischaracterised by well-developed morphological scarps, up to 60–80 mof vertical displacement, that gradually rotate towards SE (Fig. 1, cfr.Chiocci et al., 2011).

The Ripe della Naca–Piedimonte–Calatabiano system is formed ofnormal structures located along the northeast flank formed by severalmain segments. The Ripe della Naca faults (RP) consist of a couple ofWSW–ENE oriented scarps from 80 to 120 m high. The Piedimontefault (PD), WSW–ENE oriented, and the Calatabiano fault (CL), NE–SWstriking, are interpreted as regional tectonic lineaments (Finetti et al.,2005; Lentini et al., 2006). RP–PD are buried by Holocene lava flowsand truncated by the Pernicana fault segments (Azzaro et al., 2012a).Short normal faults showing a similar NE–SW trend are present alongthe Ionian coast. The continuation in the offshore of these small faultsegments is given by several well-developed morphological scarps, NE

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striking, characterised by several tens ofmeters of vertical displacement(Chiocci et al., 2011).

Recently, the relationships between the structural setting of Etna'sunstable eastern flank and the offshore morpho-lineaments of the con-tinental margin facing the volcano have been defined by Chiocci et al.(2011). In particular, the authors showed that the continental marginis affected by a wide semicircular fault system as the expression of alarge-scale retrogressive instability phenomenon that also involves thevolcano costal belt. In this new interpretation the on- and offshore por-tions of the Timpe fault system appear as a part of the flank instabilityaffecting the continental margin rather than the northernmost exten-sion of the Malta Escarpment. In addition, multichannel seismic reflec-tion data performed by Argnani et al. (2012) have evidenced that theTimpe fault system is not linked with the regional structural lineamentof the Malta Escarpment.

In this paper, an updated tectonic framework of the Etna unstableflanks is presented as a result of a multidisciplinary analysis obtainedby integrating (1) detailed geological and structural investigationsperformed for compiling the new geological and volcano-tectonicmaps of Etna volcano (Branca et al., 2011; Azzaro et al., 2012a); (2)seismotectonic data from historical ground surface rupture mapping(Azzaro, 2004); and (3) ground deformation measurements (Puglisi etal., 2001, 2004) and Permanent Scatterer (PS) data (Bonforte et al.,2011). These different typologies of datasets have been analyzed and

Fig. 2. Distribution of the historical seismicity in the Etna region from 1600 to 2010 (macroseismicthe epicentral intensities I0 according to the relationship by Azzaro et al. (2011). Inset map a) shofrom1999 to 2011 (instrumental data fromGruppo Analisi Dati Sismici, 2012); the contour of the r

correlated in order to constrain age, geometry and kinematics of thefault systems controlling the unstable flanks of Etna volcano and to bet-ter understand their complex relationships on- and offshore.

2. Seismotectonics

The structural framework given by exposed fault segments is inte-grated with seismotectonic data, which allow a more complete defini-tion of geometry of the tectonic elements and information on theiractivity in the mid-term (since the mid 19th century to date). For thepurposes of this paper, we refer to the results of the analyses carriedout for the new volcano-tectonic map of Etna (Azzaro et al., 2012a),considering the following two major features: (1) the distribution ofthemain historical and recent seismicity; and (2) the pattern of the hid-den faults, i.e. tectonic elements not recognizable through the standardapproach of the geological survey.

The location of theML≥3.2 earthquakes occurring at Etna in the lastfour centuries is shown in Fig. 2. Information provided frommacroseismic data in the long period is fairly complete and representa-tive of the major seismicity affecting the piedmont parts of the volcano,where earthquake damage in the urbanised areas is historically well-documented (Azzaro et al., 2012b). The eastern sector of the volcano,crossed by the Timpe fault system (including FF, STF, MF SLF), appearsvery active from the seismic point of view, both for number of events

data from CMTEWorking Group, 2008; Azzaro et al., 2010);magnitudes are calculated fromws the location of the earthquakes with ML≥3.2 and hypocentral depth H≤4 km, recordedift zones is in red. Fault pattern and abbreviations as in Fig. 1, C.C. indicates the central craters.

Fig. 3. Active faults from seismotectonic data (modified from Azzaro et al., 2012a). The tectonic pattern of faults with morphological expression is completed by hidden features revealed bysurface faulting (coseismic ruptures and creeping), and buried faults defined by alignments of earthquake epicentres; the contour of the rift zones is in grey. Major strike-slip components(arrows) are also shown; SVF=S. Venerina fault, other abbreviations as in Fig. 1. Stippled areas indicate the sedimentary basement.

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and the maximum intensity reached at the epicentre. The other faultsystems are characterised by moderate to strong earthquakes mainlyalong the upslope segments of the Pernicana and Tremestieri faults,and amore scattered seismicity in the south-western sector. On the con-trary, while no significant macroseismic effects are available for thewide deserted zone around the active summit craters, recent instru-mental data highlight a recurrent seismic activity along the Pernicanafault (Alparone et al., 2012b) and the rift zones, the latestmainly relatedto the seismic swarms accompanying the 2001, 2002–03 and 2008–09flank eruptions (Patanè et al., 2003; Barberi et al., 2004; Alparone etal., 2012a).

Regarding the hidden faults, this term is intended in amorphologicalsense (see Yeats et al., 1997, and references therein), and indicates sur-face breaking faults that do not exhibit a clear and permanent associatedgeomorphology. In such a case the evidence of surface faulting is pro-duced by coseismic displacement or creeping but it is generally ashort-lived feature, being visible only if the fault trace crosses man-made features (Azzaro, 1999). In practice, hidden faults are not isolatedbut represent segments of major, continuous fault zones, appearing asextensions of exposed/buried faults or as connections between faultsthat can be considered distinct from each other from a purely geologicalpoint of view. Finally, in the map of Fig. 3 we also included the blindfaults, i.e. deeper structures lacking any surface evidence but revealedby the alignment of strong historical earthquakes.

In the south-western flank, the Ragalna fault system appears as anisolated element in the structural framework of the volcano, without anapparent direct connection to the other volcano-tectonic features. Thesystem is formed by two structures that connect at their upslope termi-nation with a cusp-shaped junction (Rust et al., 2005; Neri et al., 2007).The present activity is indicated by both a remarkable aseismic creeping(Ragalna fault) with dextral oblique-slip displacements, and a moderateseismicity, with shallow events not exceeding I0 VII EMS (or ML 3.7) lo-cated along the downhill extension of the Calcerana fault (countryside

north-east of Adrano). Other earthquakes are known to have occurredalong an alignment, striking NE–SW, between the localities of Biancavillaand Ragalna (Fig. 2), which marked the pattern of a blind fault.

In the southernflank, the Tremestieri–Trecastagni faults define a tec-tonic system extending for about 10 km from the southern terminationof the S rift downslope to a fewkilometreswest of the coast-line (Azzaroet al., 2012a). Although poorly defined from the morphological point ofview – the uppermost segments are hidden and blind, respectively –

they are characterised by important dynamics with very shallow seis-micity of moderate energy (earthquakes do not exceed I0 VII EMS orML 3.7) and above all fault creep (Azzaro, 2004). These structures havea relevant role in controlling, by dextral oblique movements, the sea-ward movement of the volcano's eastern flank, and constitute a struc-tural limit of first order between the unstable sector to the north andthe relatively steady area to the south (Solaro et al., 2010; Bonforte etal., 2011, 2012a; Gambino et al., 2011).

The continuity of the Timpe system is well-evidenced over much ofthe eastern flank: near the coast, the NNW–SSE structural trend(Moscarello–Acireale–S. Leonardello faults) dissects the base of thevolcano'sflank by prevailingly verticalmovements, while in the interiora prominent feature is represented by the NW–SE striking system,which defines a right-lateral shear zone displacing the flank (Azzaro,2004). In this framework, the Fiandaca–S. Tecla–S. Venerina faultsappear to be important tectonic elements connecting the Timpe systemwith the central part of the volcano and the volcano-tectonic depressionof theValle del Bove (see the blind segment of the S. Tecla fault in Fig. 3).It should be stressed that all these faults are very active also from theseismic point of view, representing the sources of the strongest earth-quakes reported in the seismic catalogue for the last centuries (CMTEWorking Group, 2008). With a long-term behaviour (~200 years)characterised by a mean recurrence time of about 20 years forsevere/destructive events (epicentral intensity VIII≤ I0≤ IX–X EMS,corresponding to a magnitude range 4.3≤ML≤5.1 according to

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Azzaro et al., 2011), the Timpe fault system is highly relevant interms of seismic hazard, especially when short exposure times(5–30 years) are considered (Azzaro et al., 2008; 2012b).

The Ripe della Naca–Piedimonte–Calatabiano faults in the north-eastern flank, despite their outstanding morphological evidence, donot appear active in historical times given their lack of seismicity orany surface faulting (Azzaro, 2004). For this reason they are not repre-sented in the active fault map of Fig. 3. Finally, to the North thePernicana fault system decouples the entire eastern flank of the volcanofrom the NE rift as far as the sea (Tibaldi and Groppelli, 2002; Neri et al.,2004; Bonforte et al., 2007a, 2007b; Guglielmino et al., 2011b). It ex-tends for nearly 20 km and consists of twomain segments, the westernone expressed by an evident sinistral oblique fault, the eastern oneappearing as a strongly segmented left-lateral fault zone devoid of mor-phological expression (Acocella and Neri, 2005). While the primarymechanism triggering the Pernicana fault is still debated – active fordyke-induced rifting or passive for gravity-induced sliding (Acocellaet al., 2003; Walter et al., 2005; Bonforte et al., 2007b; Currenti et al.,2008, 2010) – it is widely recognised as the most active structure inthe volcano. Its important dynamics is demonstrated by the very fre-quent seismic activity occurring in the western and central sections ofthe fault (Azzaro, 1997; Alparone et al., 2012b), with strong earth-quakes reachingML 4.7 and I0 VIII EMS, and by aseismic creep in its east-ern sector, accommodating a large part of the overall displacement ofthe unstable flank (Obrizzo et al., 2001; Bonforte et al., 2007a).

3. Ground deformation

Ground deformation measurements on Etna have been carried outby EDM techniques since 1970s and by GPS since 1988 (Puglisi et al.,2001, 2004). Geodetic studies highlighted that the deformation of theeastern and south-eastern flanks of Etna is quite different from the radi-al pattern that one would expect in a central basaltic volcano (Bonforteand Puglisi, 2003, 2006; Bonaccorso et al., 2006; Houlie et al., 2006;Bonforte et al., 2011; Guglielmino et al., 2011a). GPS data showed thatthe deformation rate of the eastern side is always higher than thatexpected by assuming a simple magmatic source. The first analyticalmodel obtained before the 2001 eruption (Bonforte and Puglisi, 2003)highlighted a planar surface beneath the mobile flank at a depth ofabout 2 km b.s.l., gently dipping eastwards. Further improvements inthe network during the late 1990s (Puglisi and Bonforte, 2004;Bonforte and Puglisi, 2006), revealed a first-order rotational sliding ofthe easternflank, detecting for thefirst time the different blocksmakingup the unstable sector. The Pernicana fault and its extension as far as thecoastline (Acocella and Neri, 2005) was confirmed to represent thenorthern boundary (Bonforte and Puglisi, 2006), and it was possibleto investigate its relationship with flank dynamics (Bonforte et al.,2007a) and eruptive activity, especially along the NE rift (Bonforte etal., 2007b,c). On the southernflank, GPSmeasurements detected a grad-ual decrease and rotation of the displacements moving southwards,suggesting that the southern limit of the sliding sector was less sharpthan the northern one.

Another detachment level located at roughly sea level, wasmodelledby Bonaccorso et al. (2006) and Bonforte et al. (2008) following the2001 and 2002–2003flank eruptions. Due to thenecessity of simplifyingthe models, all geodetic data inversions resulted in only one plane at atime and the different results highlighted the complexity of the detach-ment, probably consisting of a family of non-planar surfaces inheritedfrom the lithological features of the basement (Bonforte et al., 2007a,2009; Alparone et al., 2011).

In the late 1990s, the spatial sampling of the ground deformationthrough Differential SAR Interferometry (DInSAR) analysis, alloweddetecting unexpected dynamics especially on the lower south-easternside of the volcano (Borgia et al., 2000; Froger et al., 2001; Solaroet al., 2010; Bonforte et al., 2011, 2012a; Gambino et al., 2011) and,combined with a geochemical approach (Neri et al., 2007), recognising

the uphill extension of the Ragalna fault as far as the summit part of thevolcano. Further improvements were obtained from the PermanentScatterer (PS) and Small BAselines Subset (SBAS) approaches (Ferretti,et al., 2001; Berardino et al., 2002), based on time series analyses ofInSAR data aimed at removing the main noise sources affecting DInSARimages (Zebker and Villasenor, 1992; Massonnet and Feigl, 1995;Zebker, et al., 1997; Bonforte et al., 2001; Onn and Zebker, 2006). Thistechnique enables identifying fault displacements even in urban areaswhere evidence is difficult to detect by traditional field surveys. Grounddeformation maps resulting from time series analysis allowed Solaro etal. (2010) and Bonforte et al. (2011) to investigate kinematics of thesliding flank, and to recognise important features interpreted as discon-tinuities in the ground velocity field.

Zoning of the sliding sector resulting from the previous GPS and SARstudies is shown in Fig. 4, with the main tectonic discontinuities, mark-ing abrupt changes in the ground velocity field, that identify five maindomains: (1) the NE block, bordered to the North by PF andcharacterised by the highest deformation velocities; ground velocityprogressively diminishes toward South, with a clockwise rotation ofthe vectors defining a (2) central block between MF and STF; (3) theGiarre wedge, on the easternmost corner of the volcano east of theTimpe system; (4) the Medium-East block, bounded by STF and TCFand (5) a relatively less active SE block, moving southwards, borderedby a hidden tectonic lineament evident also from soil gas surveys(Bonforte et al., 2012b), developing throughout the southern flankfrom Belpasso to Ognina (BOL). GPS time series plots of some stationslying on the different domains, are also reported in Fig. 4. It is clearthat each domain is characterised by different dynamics in terms ofvelocity and displacements but all exhibit a discontinuous movement:the fairly constant mid-term (decennial) seaward sliding is interruptedby sudden short-term (months to year) accelerations related to flankeruptions (Acocella et al., 2003; Bonforte et al., 2007a; Puglisi et al.,2008; Bonforte et al., 2009).

4. Kinematic model of the sliding flank

Both the complexity of the kinematics of the unstable sector as wellas the extent of the details of the several fault systems decoupling theentire sector in different kinematic blocks have increased since thefirst simple conceptual models (Fig. 5). After the first insights aboutflank instability on Etna from field evidences (see introduction) andfrom analogue models (Merle and Borgia, 1996), new hypotheses onthe geometry of the sliding sectorwere proposed based on instrumentaldata as DInSAR (Froger et al., 2001) and GPS (Bonforte and Puglisi,2003). Furthermore, the first analytical models were proposed whenground deformation instrumental data became available, albeit ononly a few points lying on the unstable flank (Bonforte and Puglisi,2003; Puglisi and Bonforte, 2004; Houlie et al., 2006). These modelsgave the first evidence of a detachment beneath the eastern side ofthe volcano, more recently confirmed by comparing seismic and geo-detic data (Bonforte et al., 2009; Alparone et al., 2011) and providedthe first estimations of sliding rates. Later on, these models were pro-gressively refined following the improvements of the geodetic net-works that also revealed a very first arrangement of this flank indifferent sliding blocks (Bonforte and Puglisi, 2006). Integration ofGPS dense network and DInSAR data also gave evidence of possiblemultiple detachment surfaces at different levels (Bonaccorso et al.,2006), that can be triggered by intrusive events able to promote flankinstability, such as the 2001 and 2002 ones (Acocella et al., 2003;Bonforte et al., 2007a, 2007c, 2009). Besides the overall sliding andthe depth of the detachment, further details in the complex arrange-ment of several blocks making up the unstable flank have been contin-uously furnished by field evidences during particularly high ratesperiods (Rust et al., 2005). More sophisticated remote sensing data pro-cessing (Solaro et al., 2010; Bonforte et al., 2011) has highlighted hiddenlineaments not revealed by field data, completing the information from

Fig. 4. Summary of ground deformationmeasurements at Etna. Brown lines indicate the main features detected by SAR data, associated with the considered fault systems (abbreviations as inFigs. 1 and 2). Themain blocks composing the sliding flank, indicated by different colours in the centralmap, are redrawn from Bonforte et al. (2011). Arrows indicate themean velocities of theGPS stations, whose time series are reported in the plots (coloured according to the relevant block): CRI, Mt. Crisimo; EGIA, Giarre; EMSC, Mascalucia; ESRV, Scorciavacca; ESVN, S. Venerina;ETAC, Torre Archirafi; FOPE, Fossa del Pero; PDAP, Piano d'Api; PISA, Pisano; SGBO, S. Giovanni Bosco; STAZ, Stazzo.

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the detailed volcano-tectonicmapping (Azzaro et al., 2012a) and the re-cent analogue model by Norini and Acocella (2011).

In the model we are proposing, we integrated the tectonic elementsrecognised in the field according to standard procedures (see themeth-odological approach in Azzaro et al., 2012a) with the lineaments in-ferred from geophysical data (see Bonforte et al., 2009, 2011; Chiocciet al., 2011), including information on recent and long-term slip-rates,i.e. from the late Pleistocene to Holocene (100–3.9 ka). This time inter-val can indeed be considered representative for defining the evolution ofa complex, large-scale geologic process such as flank instability. Thesefeatures are used to define the overall geometry of the model both on-shore and offshore, and to fix the boundaries of the unstable area.Fig. 6 shows the main structures involved in the flank instability pro-cesses, mapped with simplified geometries (linear elements, no bend-ing) as they are intended to represent first order elements at the scaleof the entire volcanic edifice. The tectonic elements controlling theflank instability processes can be identified with the following groupsof structures: (1) the Ragalna fault (RF) and the Belpasso-Ognina linea-ment (BOL) that, together with the Tremestieri (TMF) and Trecastagni

faults (TCF), bound to the south the unstable flank by extensionalmove-ments with a minor dextral component; (2) the Timpe fault system(formed by FF, ARF, STF, SVF, MF, SLF), which dissects the eastern sideof the volcano and accommodates large strains induced by the exten-sional, seaward sliding of this sector by dip-slip and right-lateral obliquedisplacements; and finally (3) the Pernicana fault (PF), which repre-sents the main shear zone of Etna decoupling to the north the mobilesector by a oblique, left-lateral strike-slip. In the scheme,we also consid-ered the lineaments resulting from DInSAR data in the upper part of thevolcano, which essentially follow the axes of the volcanic NE and S riftzones and close the unstable area to the west. They connect the summitcraters with the upslope tips of the Ragalna–Tremestieri–Trecastagni–Pernicana faults, defining the unstable sector onshore. Concerning themorphostructural setting of the Etna offshore proposed by Chiocci etal. (2011), the presence of instability elements along the continentalmargin is evident between the Fiumefreddo valley to the north andthe Catania canyon to the south (Fig. 6). Although these two relevantmorphological features appear located near the termination of thePernicana fault and Belpasso-Ognina lineament, no structural linking

Fig. 5.Mainmodels offlank instability at Etna proposed in the literature during the past 20 years: a) LoGiudice and Rasà, 1992; b) Borgia et al., 1992; c) Rust andNeri, 1996; d) Rasà et al., 1996;e) Merle and Borgia, 1996; f) Bonforte and Puglisi, 2003, 2006; g) Rust et al., 2005; h) Bonaccorso et al., 2006.

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has been found by Argnani et al. (2012). The semicircular fault systemaffecting the continental margin, while appearing linked with somefault segments of the Timpe system, represents very shallow secondorder structures that passively accommodate the movement of thislarge sector of the continental margin toward the abyssal plain.

Fig. 6 also shows two sketch sections crossing the central part of thevolcano. Themost outstanding features playing a role in flank instability,are: (1) the high velocity body (HVB) confined in the central-eastern partof the volcano at a depth between 3 and 9 km b.s.l. (Vp 5.6–6.4 km/s),which is interpreted as high-density cumulates fractionated during

Fig. 6. Pattern of themain volcano-tectonic elements proposed formodelling dynamics of the unstable sector of Etna; offshore features are redrawn fromChiocci et al. (2011). Legend: 1, faultsandmajor strike-slip components; 2, lineaments andother structures delimiting the study area; 3,mainmorphological scarps; 4, landslide scarp; 5, rift zones; 6, activedetachmentplanedefinedbygrounddeformationdata (Bonaccorso et al., 2006; Bonforte andPuglisi, 2006). Geometric andkinematic parameters are reported in Table 1; coordinates andGISmetadata are provided in theelectronic supplementary material. Cross-sections show the high velocity body (HVB) defined by seismic tomography (Chiarabba et al., 2004); fault abbreviations as in Fig. 1.

Table 1Geometric and kinematic parameters of active faults proposed for modelling kinematics of the unstable sector of Etna in an elastic half-space medium (WGS84 coordinates are pro-vided in the electronic supplementary material). Slip-rates represent long- and short-term estimations, calculated from geological and historical data, respectively (Azzaro et al.,2012a). Components of movement: (v) vertical, (d) dextral, (s) sinistral. Note that the most critical definition concerns the dip and width of the faults, which have been inferredfrom the inversion of ground deformation data; for this reason, in some cases they are not available.

Faults Length(km)

Strike Dip Width(km)

Dominant faulting style Long-term slip-rates(mm/yr)

Short-term slip-rates(mm/yr)

Pernicana (PF) 19.4 50°–115°a 60°–82°Sa 2.6–1.6a Oblique, sinistral strike-slip 3.3–5.2 (v) 10 (v)–28 (s)

Ragalna (RF) 4.90°

60°NE–89°E 2.7–1.6b Dextral, oblique slip 1.3–1.5 (v) 3.7 (v)–3.4 (d)60°

Tremestieri (TMF) 7.2 135° 60°–89°NE 1.6b Dextral, oblique slip 2.5 (v) 4.0 (v)

Trecastagni (TCF) 7150°

60°–89°NE 1.6b Dextral, oblique slip 2.5 (v) 4.0 (v)100°

Fiandaca (FF) 13.8140°

Dextral, oblique slip 1.0 (v) 2.0 (v)5°

Acireale (ARF) 6.2 175° Normal, with right-lateral components 1.3–4.3 (v)S. Tecla (STF) 7.6 140° Normal, with right-lateral components 4.3 (v) 3.0–10 (v)S. Venerina (SVF) 5.5 135° Normal, with right-lateral componentsMoscarello (MF) 11.3 165° 89°E 3.5 Normal with minor dextral slip 1.4–2.7 (v) 3.0 (v)S. Leonardello (SLF) 9 165° 89°E 3.5 Normal with minor dextral slip 2.6 (v) 5.0 (v)Belpasso-Ogninac (BOL) 22 125° 60°–89°NE 1.6b Dextral, oblique slip 4 (v)–5 (d)

a End-member values varying from west to east (Bonforte et al., 2007b).b Depth, from north to south, of the sliding surface modelled in Bonforte and Puglisi (2006).c Tectonic lineament revealed by InSAR data (Bonforte et al., 2011) and CO2 soil degassing (Bonforte et al., 2012b).

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repeated intrusions (Chiarabba et al., 2004) or fossil magma chambers ofancientMt. Etna (Laigle et al., 2000); (2) the presence of both extensionaland compressive domains at different crustal levels separated by (3)two E-dipping planar surfaces of detachment – most likely a family ofdiscontinuities – located at a depth ranging from 2 to 4 km b.s.l(Bonforte and Puglisi, 2003). According to the authors, before an erup-tion (recharging period) flank instability is favoured by the pressuriza-tion of the deeper feeding system of the volcano, which activates adeep-rooted detachment at the top of the rigid body (see section W–Ein Fig. 6). The seismic and geodetic strains (Alparone et al., 2011) relatedto the extensional dynamics abruptly disappear at a depth of 2–4 kmb.s.l., i.e. the level of the deeper sliding surface. Beneath this level, thestress field is unequivocally driven by the pressurization of the feedingsystem, while above the detachment plane the inflation may induceextension, rarefaction and depressurization of the upper central area.These, in turn, facilitate the ascent ofmagma towards the summit craters(Carbone et al., 2009; Bonaccorso et al., 2011), triggering further slidingabove the shallower detachment. The activation of this detachmentplane during shallow intrusions leading to flank eruptions (i.e. the2001 to 2004 case-histories in Bonaccorso et al., 2006 and Bonforteet al., 2009), is marked by acceleration periods that are evident in theGPS time series in Fig. 4.

The boundaries of the sliding sector are shown in the section N–Sof Fig. 6. These structures join with the detachment plane at a depthof 2–4 km b.s.l., corresponding also to the maximum depth of theearthquakes along the Pernicana fault (Alparone et al., 2012b) and tothe detachment plane also detected by magnetotelluric surveys(Siniscalchi et al., 2012). The overall geometry of the unstable sectoris therefore formed by a family of shallow listric faults connectedwith a sole fault. This setting also extends to the Etna offshore withthe semicircular pattern of skin-deep structures.

In conclusion, although the overall tectonic frameworkmight appearconceptually easy to apply to an analytical model (Table 1), modellingflank dynamics is indeed complicated by the overall clockwise rotationof the north-eastern flank, by the presence of rotational components(westwards tilt) of the different sub-blocks making up the unstablearea between the Acireale and Fiandaca faults (ARF and FF, see alsoAzzaro et al., 2012a for details) as well as by at least two detachmentplanes responsible for the instability processes. Rotational movementsare also evident on the south-eastern part of the sliding flank fromground deformation data reported in Bonforte et al. (2011), revealingthe westwards tilt of each block; also at a wider scale, the entire south-ern and south-eastern flanks show an overall rotational behaviour withan outstanding active uplift of the southern periphery of the volcano.Regarding the structures delimiting the unstable sector, the very lowdislocation of the Holocene volcanic products (Branca et al., 2011;Azzaro et al., 2012a) compared with the ground deformation measure-ments, and the immature morphological development of the Pernicanaand Tremestieri–Trecastagni fault segments toward the coast (Acocellaand Neri, 2005; Azzaro et al., 2012a), suggest that their seaward migra-tion occurred in more recent times. This evidence is similar to the for-mation of lateral ramps in a rotational landslide, whose verticaldisplacement is progressively reduced toward the front due to the topo-graphic inversion produced by the general tilting.

5. Concluding remarks

In this work, geological and geophysical information has been ana-lyzed and correlated in order to define an updated and comprehensivekinematic model of the sliding flank of Etna volcano. Starting from thenew geological and volcano-tectonic maps, we recognised the main tec-tonic structures dissecting the unstable sector of the volcanoanddefinedtheir long-term slip rates (100 to 3.9 ka); geometric relationships withthe active volcanic rift zones have also been taken into account. Themost recent seismotectonic data have been used to identify the faults ac-tive in the mid-term (last 200 a), to characterise the fault behaviour

(stick-slip vs stable-sliding) and the prevailing faulting style. The overalltectonic framework has been defined by considering also the hiddenfault segments, which constitute discrete elements of more extendedstructures, as well as the morphostructures detected in the offshore.Ground deformation data from GPS and DInSAR measurements havebeen exploited to highlight fault zones showing significant strain inthe short-term (last 20 a) but also dynamics along lineaments not de-tectable by other methods, and to define the main kinematic domainscomposing the sliding flank of the volcano. Geodetic data has also pro-vided indications on the geometry and depth of the detachment planes,marking the upper and lower limits of a transition zone between thesliding portion and the stable one.

The integration of all the information obtained by the different ap-proaches led to a final comprehensive model of the unstable sector,which contains the basic input parameters for modelling flank dynamicsat Etna. The first-order overall sliding kinematics are complicated by ageneral horizontal (clockwise) rotation of the entire north-eastern sectorand a general vertical (westward tilt) rotation of the south-eastern one;furthermore, a second-order westward tilt of each block making up thesouth-eastern sector has been evidenced by ground deformation data,suggesting a listric geometry of the faults. In order to facilitate the appli-cation of analytical models, the 3D geometry of the structures is simpli-fied as rectangular planes and each element provided with the basicinput parameters to be used in the computations. The considered tectonicelements identify the boundaries of the whole sliding sector (Pernicana,Ragalna, Tremestieri–Trecastagni and Belpasso-Ognina systems) andthe main structures of the Timpe systems dissecting it into blocks withdifferent kinematics. The defineddomains are characterised bydiscontin-uous dynamics: the fairly constant seawardmovement (i.e. themid-termflank sliding) is interrupted by sudden, short-lasting accelerations relatedtoflank eruptions. At depth, the activation of the detachment planes driv-ing flank instability is favoured by the pressurization of the feeding sys-tem or by shallow intrusions leading to flank eruptions.

The immature degree of development of the faults delimiting the un-stable sector, suggests that their propagation occurred in more recenttimes with respect that of the Timpe fault system. Furthermore, this evi-dences a retrogressive upwardpropagation of the instability phenomena,which on thewhole involve both the continental margin and the easternsector of Etna volcano.

Acknowledgment

This work was funded by the Italian Dipartimento della ProtezioneCivile in the frame of the 2007–2009 agreement with Istituto Nazionaledi Geofisica e Vulcanologia— INGV, project V4: “Hazard connected to theflank dynamics of Etna”.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.jvolgeores.2012.10.001.

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